^^^^
yeah, sounds to me like their using Gamebryo as a starting point for the new engine and not necessary a slightly upgrade version. i can imagine that it has to do with how the game handles the open world environment since there were few issues in that regard with either Fallout 3 or New Vegas. everything else with that engine needs a significant overhaul to keep up with new engines coming from iD and Crytek (particularly Crytek given that both previous Crytek engines handled large open environments very well).